This invention relates generally to a counter, and more particularly to an asynchronous type counter.
Counters are generally classified as either synchronous or asynchronous. As shown in FIG. 21(a), a conventional synchronous counter 2100 includes two or more flip-flops (FF's) in which the clock terminals of each flip-flop are connected to a common clock input. FIG. 21(b) illustrates a conventional asynchronous counter 2150 in which a clock terminals of at least one of the flip-flops is not connected to the clock terminals, of any of the other flip-flops. Asynchronous counter 2150 is also commonly referred to as a ripple counter, that is, the clock is derived from a previous stage output.
Synchronous counter 2100 can assume sixteen different values (i.e., a 16-ary counter). Asynchronous counter 2150 has a far simpler circuit configuration and requires far fewer transistors than synchronous counter 2100. Asynchronous counter 2150 can assume 2.sup.n values (i.e., 2.sup.n -ary counter). Until recently, asynchronous type counters functioning as other than a 2.sup.n -ary counter were not commonly used.
A conventional synchronous counter 2200 and a conventional asynchronous counter 2250 functioning as other than 2.sup.n -ary counters are shown in FIGS. 22(a) and 22(b), respectively. Counters 2200 and 2250 are each 13-ary counters (i.e., can assume 13 different values). Asynchronous counter 2250, which resets the first four counters when the Q output of the fifth counter is at a low logic level, requires far fewer transistors than synchronous counter 2200. Far more transistors, however, are required by asynchronous counter 2250 than are used in the 16-ary asynchronous counter 2150. In particular, a conventional asynchronous counter functioning as other than a 2.sup.n -ary counter requires far too many transistors. Furthermore, since the manufacturing cost of the counter is proportional to the number of gates required by a counter, the manufacturing cost of an asynchronous counter is far too expensive.
Accordingly, it is desirable to provide an asynchronous counter which requires far fewer transistors and therefore is less costly to manufacture than a conventional asynchronous counter in producing other than 2.sup.n count values.